1. Field of the Invention
The present invention relates to antennas and more particularly to structurally-embedded conformal antennas.
2. Brief Description of Prior Developments
A polarization-diverse receiving system for modern low-RCS airborne platforms requires a physically small, low profile, low RCS, broadband and dual independent polarization antenna. It is believed, however, that there is currently no such an antenna meeting all the needs that systems desire, especially over the VHF/UHF frequency spectrum due to the nature of long wavelengths over these bands.
A standard multi-polarization cross-loop (X-loop) antenna, as shown in FIG. 1(a), is used on similar applications when there are no RCS constraints or aerodynamic drag constraints. A couple of new types of flush-mounted and cavity-backed antennas were designed to address the aerodynamic drag. These antennas are Archimedean Spiral FIG. 1(b), Sinuous FIG. 1(c), and Serrated-edge slot FIG. 1(d) antenna, as shown below. These cavity-backed antennas are all loaded with lossy absorbing material to dampen the high Q resonant cavity modes.
The disadvantage of X-loop, as shown in FIG. 1(a), is that it has higher aerodynamic drag even with the use of an aero radome and it does not address RCS factors at all. The disadvantage of Archimedean Spiral antenna, as shown in FIG. 1(b), is that the linear polarization components rotate with frequency, which complicates antenna calibration on certain applications. In addition, the Spiral antenna suffers large out-of-band RCS as the density of slots result in a large impedance discontinuity between the spiral aperture and the ground plane. Although the sinuous antenna, as shown in FIG. 1(c), resolves the frequency-dependent linear polarization issues of the Archimedean Spiral, the density of slots still generates a significant out-of-band RCS. There are two disadvantages to the Serrated-edge slot antenna design shown in FIG. 1(d). The first disadvantage is that the antenna matching is solely accomplished by absorbing material thus reducing radiation efficiency. The second disadvantage is that the multiple lobes appear in the patterns at the high end of operating frequency band because the currents flow freely over the entire width of the patch. Thus, the useable operating frequency band is limited.
No existing antenna is known to be capable of providing dual independent polarization and broadband VHF/HUF operations with low RCS characteristics while the antenna is electrically small, low profile and conformal flush-mountable.
A need, therefore, exists for an antenna which overcomes the disadvantages of the prior art.